A spark plug for an internal combustion engine such as an automotive engine is provided with, for example, a center electrode, an insulator provided at the outside of the center electrode, a tubular metallic shell provided at the outside of the insulator and a ground electrode with a base end portion joined to a front end surface of the above-described metallic shell. The ground electrode has a substantially rectangular cross section and is disposed so that its front end portion inner surface faces a front end portion of the above-described center electrode, whereby a spark discharge gap is formed between the front end portion of the center electrode and the front end portion of the ground electrode.
The metallic shell is formed at an outer circumferential surface with an unshown thread portion. The spark plug is threadedly attached to a spark plug hole having a female thread and formed in a cylinder head of an engine. In the meantime, in case in an installed state of the spark plug, the mixture gas has such a positional relationship of lashing against a back face of the ground electrode, there is a fear of the ground electrode obstructing flow of a mixture gas into a spark discharge gap. As a result, there is a fear of variations in the ignitability being caused.
In contrast to this, there is a technique that in a spark plug of the type having two or more ground electrodes, each ground electrode is formed into a cylindrical shape having a substantially circular cross section (refer to, for example, Patent Document 1). By making, in this manner, the cross section be substantially circular-shaped, the mixture gas is hard to come off from the ground electrode but is caused to turn around the ground electrode into the inside thereof even when the mixture gas has such a positional relationship of lashing against the back face of the ground electrode, thus enabling the mixture gas to reach the spark discharge gap easily.
Further, there is a technique that the cross section of the ground electrode is formed into a substantially trapezoidal shape (refer to, for example, Patent Document 2). By making, in this manner, the cross section be substantially trapezoidal-shaped, it can be said that the mixture gas reaches the spark discharge gap more easily as compared with the case the cross section is rectangular.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 11-121142
Patent Document 2: Unexamined Japanese Patent Application Publication No. 5-13146